Re: [PATCH 2/4] perf/x86/intel: Don't context switch DS_AREA (and PEBS config) if PEBS is unused

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 14, 2026, Jim Mattson wrote:
> On Tue, Apr 14, 2026 at 12:14 PM Sean Christopherson <seanjc@google.com> wrote:
> > @@ -5065,6 +5056,28 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
> >         if (pebs_mask & ~cpuc->intel_ctrl_guest_mask)
> >                 guest_pebs_mask = 0;
> >
> > +       /*
> > +        * Context switch DS_AREA and PEBS_DATA_CFG if and only if PEBS will be
> > +        * active in the guest; if no records will be generated while the guest
> > +        * is running, then running with host values is safe (see above).
> > +        */
> > +       if (!guest_pebs_mask)
> > +               return arr;
> 
> I think this has an unintended side effect. If DS_AREA and
> PEBS_DATA_CFG were previously listed (because pebs_guest_mask was
> previously non-zero), KVM will leave the stale entries in the MSR-load
> lists.
> 
> KVM only clears MSR-load list entries for enumerated MSRs with
> matching guest and host values.

Argh, right.  This as a delta change I guess?  I'd love to overhaul the interface,
but reworking KVM to play nice with core_guest_get_msrs() would be much more
difficult.

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 20a153aa33cb..3244d5589981 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -5059,22 +5059,20 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
        /*
         * Context switch DS_AREA and PEBS_DATA_CFG if and only if PEBS will be
         * active in the guest; if no records will be generated while the guest
-        * is running, then running with host values is safe (see above).
+        * is running, then simply keep the host values resident in hardware.
         */
-       if (!guest_pebs_mask)
-               return arr;
-
        arr[(*nr)++] = (struct perf_guest_switch_msr){
                .msr = MSR_IA32_DS_AREA,
                .host = (unsigned long)cpuc->ds,
-               .guest = kvm_pmu->ds_area,
+               .guest = guest_pebs_mask ? kvm_pmu->ds_area : (unsigned long)cpuc->ds,
        };
 
        if (x86_pmu.intel_cap.pebs_baseline) {
                arr[(*nr)++] = (struct perf_guest_switch_msr){
                        .msr = MSR_PEBS_DATA_CFG,
                        .host = cpuc->active_pebs_data_cfg,
-                       .guest = kvm_pmu->pebs_data_cfg,
+                       .guest = guest_pebs_mask ? kvm_pmu->data_cfg :
+                                                  cpuc->active_pebs_data_cfg,
                };
        }